This invention relates to the reduction of noise and crosstalk in time division switching systems and in particular, the reduction of noise and crosstalk in pulse amplitude modulated time division switching systems.
Time division switching systems are typically arranged so that a plurality of station sets can be connected in pairs via a common bus or highway. This is accomplished by connecting each station to the common bus by a time division switch that is controlled by a master time-slot control circuit. When a communication channel is set up between stations, the master time-slot control circuit assigns a time slot to the connection and during that time slot the two stations involved are coupled to the common bus by the time division switches. At the end of the time slot, the switches are disabled to uncouple the stations from the common bus and another pair of switches are enabled to provide another communication path through the system between another pair of stations. In this manner, many communications connections may be set up through the system sequentially.
However, noise and crosstalk are produced in switching systems due to non-ideal characteristics of the time division switches. In particular, many switching systems utilize transistor switches in order to couple and uncouple the communication stations from the common bus. These transistor switches present a finite capacitance across the transistor due to leakage and packaging capacitances even when the transistor is in the off-state. Consequently, a particular communications channel is coupled to the common bus by the leakage capacitances even when its transistor switch is off and energy which is present on the common bus due to other communications connections is therefore introduced into the channel. Certain arrangements of time division switches have been developed which tend to minimize this coupling effect. However, these arrangements tend to be complicated and expensive.
Therefore, there appears to be a need for a method of reducing crosstalk and noise signals which result from time division switch stray capacitances. Accordingly, it is an object of the present invention to provide such a method which substantially reduces noise and crosstalk in a pulse amplitude modulated time division switching system.
It is a further object of the present invention to reduce noise and crosstalk without requiring substantial modifications in the time division switch.